Please see additional minor edits as requested by one of the reviewers. In particular, I would tend to agree that claiming to report 'structures' using only EI-MS is questionable, given that a conventional analytical chemistry publication would typically require multiple techniques to make this claim. I would also encourage the authors to address the point about atmospheric implications because this is essential to publication in ACP.

I thank the authors for addressing the comments, however the very small edits still do not completely deal with the concerns. The authors should address the following points before this paper can be published:

* I thank the authors for tempering their language when referring to proposed molecular structures, however they authors should take care when using words like 'probably' because this word implies some kind of statistical likelihood. In this instance, the molecules are based on a combination of previous literature and the authors' judgement, so the text should more clearly reflect this. In particular, the opening sentence of the abstract would be more appropriate as: "The molecular compositions of polar organic compounds (POCs) in particles emitted from various vessels and excavators were characterized using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) and possible molecular structures proposed."
* The final paragraph of the discussions and conclusions section is vague and the quality of English is so low as to be unreadable. This must be rewritten.
* The title states that the paper will cover "implications for source apportionment" yet this is still not adequately discussed. To whit, the word 'apportionment' appears nowhere in the body of the text. In order for this paper to be considered in-scope for ACP, this must be discussed properly, giving examples of the type of source apportionment work that this work would influence. This could be added to the revised discussion section but I would expect more than one paragraph would be required to do this.

In addition, I spotted the following typos in the process of checking this version, although there may be others, so I would recommend the authors perform an additional proofread:

Page 18, line 9: Change 'probable' to 'probably'.
Page 25, line 2: Change 'om' to 'on'

Thank you for addressing the various comments. I now find this version to be acceptable for publication. I should note that the final discussion section dealing with the use of markers greatly improves the applicability of the paper and with this addition of this discussion, so with this addition, I would have found retaining the reference to 'source apportionment' in the title acceptable, so the authors can change the title back if they so wish.